Numerous methods are known for detecting combustion misfires in cylinders. Of these methods, only those are of interest in the following which operate with so-called rough-running variables. These variables describe essentially speed fluctuations of the engine. The greater that the engine speed fluctuates because of misfires, the greater, as a rule, is the value of the rough-running variable. If the rough-running value exceeds a threshold value, then this is an indication that a misfire is present in the affected cylinder. The threshold value is typically read out of a characteristic field in dependence upon current values of operating variables.
An overview of various systems (methods and arrangements) for detecting combustion misfires is provided in U.S. patent application Ser. No. 818,884, filed Jan. 10, 1992, now abandon which is incorporated herein by reference. It is here noted that the rough-running values typically include a base term and a correction term. The base term is, for example, the difference between the time spans in which specific angular ranges are passed over during the current combustion stroke and during the immediately previous combustion stroke. respectively. This difference is zero if the engine rotates uniformly. Deviations from zero occur with misfires but also during accelerations or decelerations. The correction term serves to compensate for those kinds of disturbance effects such as accelerations or decelerations. The correction term is, for example, the difference of the time spans in which during the last and the next-to-last combustion stroke, the above-mentioned angular ranges were passed over. If a constant acceleration or deceleration is present without misfires, then the above-mentioned differences, which form the base term or correction term, are equal and it is for this reason that the rough-running value is zero if the rough-running variable is formed in that the correction term is subtracted from the base term. In order to be able to also compensate for more complicated disturbance cases as precisely as possible in addition to constant accelerations or decelerations, correction terms are often computed in a more complicated manner with especially mean formation being of significant advantage. The correction terms can have multiple components in order to consider several disturbances.
Notwithstanding the complexity to which the computation of values of rough-running variables is taken, erroneous detections are always present in practice, that is, either a trouble-free operating cylinder is assigned a misfire or a defectively combusting cylinder is not detected as such. These erroneous detections occur especially when there are multiple misfires.